Wireless power transfer techniques are increasingly being used to transfer power from electrical power sources to a wide range of devices from small hand-held consumer electronics devices, such as mobile phones and tablets requiring a few watts of power, to electric vehicles requiring kilowatts of power. In addition to the convenience of not having to plug in a device to power it or recharge its batteries, the absence of wires and cables makes for tidier desks and parking spaces, while reducing clutter and hazards such as trip and shock hazards. In some systems, power may be transferred wirelessly via a capacitive coupling or an inductive coupling, both of which offer advantages over resistive (e.g., wired) coupling for the purpose of supplying power to a device.
Wireless power transfer systems can be designed to operate at fixed or variable frequencies. in addition, wireless power transfer systems can be designed to deliver anything from a few watts of power to many kilowatts across a gap between plates of a capacitor or magnetic coils coupling a power source and a load. Typically the gap is an air gap. The energy thus delivered may be used, for example, to power an electronics circuit, to drive an electric motor in an electric vehicle, or to charge batteries in the electronics circuit or vehicle. Powering a cell phone or charging its battery requires a few watts, whereas powering the motor in an electric vehicle or charging the battery requires several kilowatts. The larger the battery circuit or motor, or the faster the battery is required to charge, the more power is needed to be transferred across the air gap.
Wireless power transfer techniques have developed in different fields of technology, which has resulted in different terms being used to describe essentially the same element or component. Such terms as ‘magnetic resonance,’ ‘magnetic coupling,’ ‘magnetic induction,’ ‘inductive power transfer’, and ‘resonant inductive power transfer’ are common. Although there are differences, these terms are generally used broadly and interchangeably to refer to systems that transfer power from a source across an air gap to a load by way of a magnetic field.
Similarly, such terms as ‘magnetic coils’, ‘induction coils’, and ‘antennas’ are used essentially interchangeably to describe the parts of a wireless power system that transfer energy across the air gap. More precisely, these terms refer to elements that are coils rather than antennas because at typical operating frequencies they transfer energy in the near field where the magnetic field is dominant. Antennas are designed with an electromagnetic field in mind, which forms once radiated energy passes from beyond the near field to the far field. Where the near field ends and the far field begins depends on characteristics of the transmitting device (e.g., coil or antenna). For wireless power transfer applications, an exact definition is usually unnecessary because the size of the air gap and the frequencies that the system operates places it firmly in the near field. Nevertheless, the aforementioned ‘magnetic coils,’ ‘induction coils’, and ‘antennas’ are similarly used interchangeably by those active in the art of wireless power transfer.
Wireless power transfer systems may use magnetic coils either alone or coupled with other tuned or tunable elements. In electric vehicle power transfer applications, a base power-transfer apparatus may contain a coil in combination with associated driving electronics or it may contain the coil with some or all the associated electronics being provided in a separate enclosure. Either way, the coil in the base power-transfer apparatus is used to transmit power via a magnetic field. Similarly, a vehicle power-transfer apparatus may contain a coil in combination with associated control electronics or it may contain the coil with some or all the associated electronics being provided in a separate enclosure. Either way, the coil in the vehicle power-transfer system is used to receive power via a magnetic field.
The term ‘power-transfer apparatus’ is used herein in a variety of different ways. It is used to identify an enclosure containing the coil, e.g., the ‘base power-transfer apparatus’ or the ‘vehicle power-transfer apparatus.’ It is also used to identify the coil itself. The term ‘power-transfer apparatus’ is used herein primarily with reference to the units for the ground (e.g., base) and for the vehicle with clarification where necessary, e.g., where the use of the term is not self-evident from the context of its use.